Polymerizable cationic viscoelastic monomer fluids

ABSTRACT

The synthesis of a novel family of cationic monomers capable of forming viscoelastic fluids are described within. These monomers are formed primarily through the reaction of allyl halides and dimethyl alkylamines. The rheological properties of the fluids can be dramatically modified with changes in alkyl length and/or counterion structure. In addition, it is noted that these viscoelastic fluids have properties quite similar to conventional polymer solutions even though the former structures are held together through physical forces and not through chemical bonding as in the latter materials. In addition, the properties of this unique family of cationic monomers are capable of forming viscoelastic fluids in high brine solutions. Even though the counterion attached to the monomer contributes greatly to the rheological properties, addition of soluble acid, bases and salts do not destroy (through ion exchange mechanisms) the viscoelastic properties of these fluids.

FIELD OF THE INVENTION

The present invention describes the synthesis of a novel family ofcationic monomers capable of forming viscoelastic fluids. These monomersare formed primarily through the reaction of allyl halides and dimethylalkylamines. The rheological properties of the fluids can bedramatically modified with changes in alkyl length and/or counterionstructure. In addition, it is noted that these viscoelastic fluids haveproperties quite similar to conventional polymer solutions, even thoughthe former structures are held together through physical forces and notthrough chemical bonding as in the latter materials. In addition, theproperties of this unique family of cationic monomers are capable offorming viscoelastic fluids in high brine solutions. Even though thecounterion attached to the monomer contributes greatly to therheological properties, addition of soluble acid, bases and salts do notdestroy (through ion exchange mechanisms) the viscoelastic properties ofthese fluids.

BACKGROUND OF THE INVENTION

It is well known that polymeric materials are generally considereduseful as viscosification agents when dissolved in an appropriatesolvent system. The major reason for this viscosity enhancement is dueto the very large dimensions of the individual polymer chain as comparedto the dimension of the single solvent molecules. Any increase in thesize of the polymer chain will produce a corresponding enhancement inthe viscosity of the solution. This effect is maximized when the polymeris dissolved in a "good" solvent. Therefore, in general, a solublepolymer is useful for thickening solvents, while a water soluble polymeris appropriate for increasing the viscosity of aqueous systems. Withregard to aqueous solutions, solvent soluble nonionic polymers and highcharge density sulfonated or carboxylate polyelectrolytes are quiteuseful in this regard and are commonly used materials. However, thesolution properties of the former family of materials are controlledprimarily through modification of the molecular weight of the polymerand through changes in the level of dissolved polymer. These materialsbecome especially effective at concentrations where the individualpolymer chains begin to overlap. This "transition" is commonly referredto in the literature polymer chains begin to overlap concentration orsimply C*. It should be noted that in most nonionic polymers ofcommercial interest, a relatively large amount of polymer is requiredprior to reaching C*. Therefore, this approach is undesirable from aneconomic viewpoint. Moreover, the rheological properties of many ofthese nonionic systems have been published. The results of these studiesshow that, in general, these solutions are shear thinning over all shearrates investigated.

Polyelectrolytes, on the other hand, are very useful and the mostcommonly used materials. However, the solution properties of thesematerials begin to deteriorate as low molecular additives (i.e., acids,bases or salts) are dissolved in the solution. These additives screenthe charges that are fixed along the chain backbone which results in adecrease in the dimensions of the polymer molecule. The viscositydiminishes as long as the chain continue to shrink.

It has been found previously (U.S. Pat. Nos. 4,460,758 and 4,540,496),for example, that intrapolymer complexes, composed of a nonstoichometricratio cationic and anionic monomeric units, can be useful inviscosifying aqueous solutions systems (as required in a variety of wellcontrol and workover fluids; i.e., water based drilling fluids and acidgelation systems). More importantly, these polymeric materials possesshigher viscosity in acid, base or salt solution than in thecorresponding fresh water system. Even more interesting is theobservation that these polymeric materials show a correspondingviscosity enhancement as the concentration of the dissolved acid, baseor salt is increased, even though the polyampholyte contains asubstantial amount of dissociable charge. As explained earlier, theseviscosity results are unexpected since the general tendency of chargedmacromolecules in these types of aqueous solutions shows a markeddecrease in thickening efficiency.

Furthermore, in recent years, interpolymer complexes have receivedconsiderable attention in the literature due to their interesting andunique properties. In most instances, these complexes are formed byintimately mixing aqueous solutions containing high-charge densitypolyelectrolytes possessing opposite charge. When these polymermolecules meet in solution, the interaction between oppositely chargedsites will cause the release of their associated counterions forming thecomplex. The counterions are now free to diffuse into the bulk solution.Normally, phase separation occurs upon prolonged standing in thesehigh-charged density complexes. As a result, these materials have poorviscosification properties. In previous U.S. patents it is reported thatlow-charge interpolymer complexes are soluble and effective inviscosifying aqueous solution systems. More importantly, these complexespossess a substantially higher viscosity than the correspondingindividual low-charge density copolymer components. As detailed earlier,these characteristics are unexpected since high-charge density complexesare insoluble in these conventional solution systems.

Even more interesting is the unique and unexpected result that thesesoluble interpolymer complexes are capable of enhancing the viscosity ofaqueous solutions under relatively broad shear conditions. With theseunique polymeric materials, dilatant behavior occurs in aqueous fluidswhich are of extreme technological utility. It is further noted thatunder the identical experimental conditions, the viscosity of theindividual copolymer components show the normal shear thinning behavior.

Shear thickening fluids are rare and have mostly been demonstrated insuspensions..sup.(1) Polymer solutions and melts are known, on the otherhand, to exhibit strong shear thinning behavior, while most liquids oflower molecular weight are Newtonian.

A polymer which was introduced by ICI (FM-9) as an antimisting agent forjet fuel was shown to be "progressively shear thickening". Work done byS. T. J. Peng and R. F. Landel.sup.(2) at Jet Propulsion Laboratoriesunder an FAA contract for antimisting in jet fuels showed thatsubjecting a solution of FM-9 in jet fuel at about 0.3-1.0 weight %polymer to steady shearing will produce viscous growth with time. It wasfound that the time scale and the extent of thickening can beaccelerated by either increasing the concentration or by increasing theshear rate. The composition of FM-9 is not disclosed by ICI or by otheragents working with this polymer.

Peng and Landel correlate antimisting behavior with shear thickening orwith high elongational viscosity which is exhibited by fluids that areable to demonstrate flow in a tubeless siphon..sup.(3)

However, as previously noted, polymeric materials are useful asviscosity enhancers when dissolved in the appropriate solvent system.The principal reason for this behavior is due primarily to the largevolume which a single macromolecular chain can occupy within thesolvent. An increase in the size of the chain produces a concommitantenhancement in the solution viscosity. However, when the polymer chainis placed in a shear field, segmental orientation takes place in thedirection of the shearing force. The viscosity of the fluid dramaticallydrops due to this orientation phenomena. This is a typical behavior ofmost solutions containing dissolved polymeric materials. However, if thepolymer molecule has a high molecular weight with a relatively flexiblebackbone and the solvent viscosity is sufficiently high, differentbehavior is anticipated.

It has been shown by several groups.sup.(4) that with increasing shearrates the viscosity should show a decrease, followed by a minimum valueand a subsequent increase in cases where both solvent viscosity andpolymer molecular weight are very high. This latter effect gives rise todilatant behavior. However, the above mentioned conditions required forthe appearance of shear thickening behavior in these polymeric solutionsystems are not applicable for many technologically interesting fluids.In most of the common synthetic polymers, it is difficult from asynthetic viewpoint to obtain sufficiently high molecular weight and, inaddition, most solvents (for example, water) have rather lowviscosities.

The instant invention teaches that a novel family of cationic-alkylcontaining monomers, i.e. polymerizable moieties, form large structuresin solution. The dimensions of these structures are comparable to thoseof polymeric chains. As a result, these structures, formed from thesemonomers are useful and very effective viscosifiers for aqueoussolutions. In addition, these monomers have markedly unique and improvedsolution properties, as compared to conventional water soluble polymers.These fluids formed with these monomers can adequately be described aspolymerizable cationic viscoelastic monomer fluids.

In addition, these monomers have markedly unique and improved solutionproperties in high brine environments, as compared to conventional watersoluble polymers.

These monomers are based on, but not limited to, the incorporation ofthe above cationic monomers into an aqueous fluid system.

In addition, these monomers are not incorporated into a polymer chainstructure via conventional synthetic techniques to form hydrophobicallyand associating type polymers. Very effective rheological control isassured even without the need to form a hydrophobically-associatingpolymer.

It should be noted in this regard that the use of hydrophobic groups onwater soluble polymers to enhance the rheological properties of waterbased fluids has been described. One approach to provide polyacrylamidebased systems containing hydrophobic groups is described by Bock, etal., U.S. Pat. No. 4,520,182 and 4,528,358. Water soluble acrylamidecopolymers containing a small amount of oil soluble or hydrophobicalkylacrylamide groups were found to impart efficient viscosification toaqueous fluids. Landoll, U.S. Pat. No. 4,304,902, describes copolymersof ethylene oxide with long chain epoxides which also requiredrelatively large polymer concentration (approximately 1%) for thickeningwater and required surfactants for solubility due to irregularities inthe polymerization. In a related case, U.S. Pat. No. 4,428,277, modifiednonionic cellulose ether polymers are described. Although these polymersshow enhanced viscosification relative to polymers not containinghydrophobic groups, the viscosification efficiency was very low,requiring 2 to 3 weight percent polymer to provide an enhancement. Theuse of surfactants to enable solubility and, in turn, viscosification,by a water soluble polymer containing hydrophobic groups is described byEvani, U.S. Pat. No. 4,432,881. The hydrophobic groups claimed areattached to the polymer via an acrylate linkage which is known to havepoor hydrolytic stability. In addition, the need for a surfactant toachieve solubility and thickening efficiency should make such a systemvery salt sensitive, as well as very sensitive to small changes insurfactant and polymer concentrations. Emmons, et al., U.S. Pat. No.4,395,524, teaches acrylamide copolymers as thickeners for aqueoussystems. While these polymers possess hydrophobic groups they areprepared using alcohol containing solvent which are known chain transferagents. The resulting polymers have rather low molecular weights and,thus, relatively high polymer concentrations are required to achievereasonable viscosification of water based fluids.

SUMMARY OF THE INVENTION

A process for producing a viscoelastic aqueous fluid utilizing acationic polymerizable monomer which has been discovered to yield veryefficient viscosifiers and rheological control additives for a varietyof aqueous media. The process relies on the solubilization of specificcationic monomers containing relatively long alkyl functionality withspecific anionically charged counterions. As a result, the hydrophobicmonomer is completely capable of producing a clear, uniform, homogeneoussolution even in the presence of other well-known water soluble acids,bases, salts and monomers. These solutions remains clear, uniform andhomogeneous mixture with no phase separation. These cationic typemonomers with the appropriate anionic counterion forms an unusual andunexpected morphology. These structures are rod-like micelles which arerelatively large monomer aggregates consisting of many individual singlemonomer entities. Comparison of the rheological properties of thesecationic monomer viscoelastic fluids with conventional hydrophobicallyassociating polymers (previously described) confirms that thehydrodynamic size of the former structures are comparable to those ofthe latter polymers.

The viscoelastic fluids formed are very effective viscosifiers andrheological control additives having aggregation numbers, in the form ofa rod, such that their intrinsic viscosities are greater than about 1dl/g, but due to its unusual structure they are not at all suscepitibleto shear degradation as is observed in most high molecular weightpolymers.

DETAILED DESCRIPTION OF THE INVENTION

The process of this invention overcomes the difficulties experienced inconventional polymeric viscosifiers and rheological control additives inaqueous media. In particular, it enables the efficient viscosificationof aqueous fluids without the need for a moderate or high molecularweight water soluble polymer. The particular structures formed viamonomers derived from the reaction of allyl halide withN,Ndimethylalkylamines are rod-like in nature and, as a result, aqueoussolution containing these structures have properties comparable torelatively high molecular polymers. These structures form without theutilization of any other surfactant or rod-like polymers.

The process relies on the appropriate choice of an anionic counterionwhich is attracted to the cationic charge present on thecationic-hydrophobic monomer. The counterions are usually those that arenot very soluble in water and surprisingly when attached to the cationicmonomer spontaneously forms the rod-like structures resulting inviscoelastic monomer fluids. These fluids have solution propertiescomparable to excellent water soluble polymeric viscosifiers. It shouldbe noted that dilute solutions of ionic and nonionic surfactants anddetergents usually behave as Newtonian liquids, i.e. viscoelasticbehavior is rare.

A criterion for the formation of these polymerizable cationicviscoelastic aqueous fluids can be approximated with the theoreticalconsiderations of D. J. Mitchell and B. W. Ninham. J. Chem. Soc.,Faraday Trans. 2,77, 601 (1981) and J. N. Israelachivili, D. J. Mitchelland B. W. Ninham, J. Chem. Soc., Faraday Trans. 2, 72, 1525 (1976). Thefirst consequences of packing condition of these cationic monomers withthe appropriate anionic counterion are applied under the assumptions of(1) constant volume v per monomer molecules and (2) constant headgrouparea a_(o) for a monomer molecule. In general, since the radius of therod-like structure cannot exceed a certain critical length, l_(c),roughly equal to but less than the fully extended length of thehydrocarbon, i.e. alkyl, group. It is shown by relatively simple packingconsiderations that rod-like structures (i.e. rod-like micelles) areable to form when the relationship v/a_(o) l_(c) =1/2, is approximatelyfound.

It is known that viscoelastic surfactant fluids can be formed in aqueousmedia as described by H. Rehage and H. Hoffmann, Faraday Discuss. Chem.Soc. 76, 363 (1983).

The water soluble polymerizable monomers which produce theseviscoelastic aqueous fluids of the instant invention are characterizedby the formula: ##STR1## wherein n is preferably 6 to 22, morepreferably 8 to 20 and most preferably 8 to 18. Typical, but notlimiting examples of preferred alkyl groups are hexyl, octyl, decyl,dodecyl and steryl groups. Typical, but not limiting examples ofpreferred anionic counterion structures are salicylic acid and its metalneutralized salts, salicylates, methyl salicylic acid and its metalneutralized salts and methyl salicylates.

The solution of the instant invention comprises water or water havingsalt dissolved therein and about 0.1 to about 20.0 wt. % of the watersoluble polymerizable monomer, more preferably 0.5 to about 10 wt. %,and most preferably 0.8 to 5.0 wt. %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic drawing representing the externalstructure of an individual rod-like structure formed from the cationicmonomers described within.

FIG. 2 illustrates a schematic drawing representing the internalstructure of an individual rod-like structure formed from the cationicmonomers described within.

FIG. 3 illustrates a viscosity-shear rate profile of 11868-106A monomersolution with a molar ratio of methylsalicylic acid as a function ofmonomer concentration.

FIG. 4 illustrates a viscosity-shearing time of 1 g/dl 11868-106Amonomer solution with a molar ratio of salicylic acid-solution salt.Shear rate is 0.017 sec⁻¹.

FIG. 5 illustrates a viscosity-shear rate profile of 11868-106A (0.25g/dl) solution with a molar ratio of salicylic acid-sodium saltmeasurement temperature is 25° C.

FIG. 6 illustrates a viscosity-shear rate profile of g/dl solution of11868-106A monomer with a molar ratio of sodium salt of salicyclic acidas a function of acrylamide monomer concentration.

FIG. 7 illustrates a viscosity-shear rate profile of 1 g/dl solution of11868-106C monomer with a molar ratio of sodium salt of salicylic acidas a function of acrylamide monomer concentration.

FIG. 8 illustrates a viscosity-shear rate profile of 11868-106B (1:1molar ratio of salicylic acid-sodium salt) as a function ofconcentration (g/dl).

FIG. 9 illustrates a viscosity-shear rate profile of 11868-106A monomer(1:1 molar ratio of salicylic acid-sodium salt) as a function of monomerconcentration (g/dl).

FIG. 10 illustrates a viscosity-shear rate profile of 11868-106C monomer(1:1 molar ratio of salicylic acid-sodium salt) as a function of monomerconcentration (g/dl).

FIG. 11 illustrates a viscosity-shear rate dependence of a 1.0 g/dl11868-106B monomer solution as function of various ratios of sodiumchloride to monomer, i.e. [NaCl]/[monomer].

FIG. 12 illustrates a viscosity-shear rate profile of a 1.0 g/dl11868-106A monomer solution as a function of various ratios of sodiumchloride to monomer [NaCl]/[monomer].

FIG. 13 illustrates a viscosity-shear rate dependence of 11868-106Cmonomer solution (1 g/dl) as a function of various ratios of sodiumchloride to monomer.

FIG. 14 illustrates a viscosity-shear rate profile of a 11868-106Bmonomer solution (1 g/dl) as a function of various concentrations ofcalcium chloride.

FIG. 15 illustrates a viscosity-shear rate profile of a 1.0 g/dl11868-106A monomer solution as a function of calcium chlorideconcentration (wt. %).

FIG. 16 illustrates a viscosity-shear rate profile of 1.0 g/dl11868-106C monomer solution as a function of calcium chlorideconcentration (wt. %).

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following examples explain the invention, which are by way ofillustration, but not of limitation.

EXAMPLE 1 Cationic-Hydrophobic Monomer

Three representative examples for the synthesis of the following monomerstructures to form these cationic viscoelastic monomer fluids isdescribed below: ##STR2##

In these representative examples n=15 (11868-106A), n=13 (11868-106B)and n=17 (11868-106C).

Into a small round bottom flask add the following materials together andheat to 50° C. for four hours:

    ______________________________________                                        11868-106A        14.2 g allyl bromine                                                          31.6 g N,N-dimethyl                                                           dodecylamine                                                11868-106B        14.2 allyl bromide                                                            28.3 g N,N-dimethyl                                                           tetradecylamine                                             11868-106C        14.2 g allyl bromide                                                          34.9 g N,N-dimethyl                                                           octadecylamine                                              ______________________________________                                    

the monomers are further purified through conventional analyticaltechniques. Elemental and NMR analysis confirms that the monomers arevery pure (>99%) and has, in these specific examples, the followingmolecular structure: ##STR3##

EXAMPLE 2 Preparation of Cationic Polymerizable Viscoelastic MonomerFluids

Representative examples for the preparation of cationic polymerizableviscoelastic monomer fluids containing the three above synthesizedmonomers are briefly described below.

Dissolve the monomer into water and subsequently dissolve astoichometric amount of such compounds as salicylic acid-sodium salt ormethyl salicylic acid, for example. It is noted that the requirementthat a stoichometric amount of the latter compounds be used is not anecessary requirement. A relatively broad range of these materials canbe used for effective utilization and preparation of these uniquecationic monomer viscoelastic fluids. However, stoichometric amounts ofthese compounds are used in the examples as a matter of convenienceonly. In any event, the viscosity of the fluid immediately increases toa value dependent upon the nature and concentration of the monomer andcounterion.

EXAMPLE 3

Solution Behavior

It is anticipated that the morphology, i.e. structures, that thesemonomers can assume in solution is quite complex and undoubtedly, thedynamics of these structures can change rapidly with time, temperature,stress, strain rate, pressure, ionic strength and the like. However, ourfindings strongly indicate that the monomers form relatively largerod-like structures, as idealized in FIG. 1. In the figure, the spheresrepresent both the vinylic "head group" and the cationic region of themonomer. The alkyl "tails" are embedded in the rod interior due to wellknown hydrophobic interactions (i.e. charge-charge, purelygeometrical-type rod overlappings and the like) that primarilycontributes to the unusual rheology of these viscoelastic fluids. Itshould be noted, however, that the complete verification of the solutionmorphology does not in any way detract from the utilization of the novelproperties of these viscoelastic fluids containing the above specificpolymerizable cationic monomers.

FIG. 3 shows some representative rheological properties of the abovedescribed cationic monomer viscoelastic fluids. An examination of thisdata clearly confirms:

1. At low monomer concentrations, very dramatic viscosity values areachievable.

2. At high shear rates, shear thinning characteristics are generallyobserved.

EXAMPLE 4

1. At low shear rates, marked time dependencies of the rheologicalproperties are generally found. This is further shown in FIG. 4 in whichthe data of the 1 g/dl fluid in FIG. 3 is expanded (i.e. low shear ratedata).

2. Dilution causes a reduction in the magnitude of the rheologicalproperties, in general.

EXAMPLE 5

1. The specific shear rate where the time dependency is found to occuris a strong function of the concentration (FIG. 5).

2. These viscoelastic cationic monomer fluids show dramatic shearthickening dependent upon concentration, molecular structure, counterionstructure, temperature and the like.

3. Even though the molecular weight of the individual monomer units isquite low (>1000 g mole⁻¹, in general), the rheological properties ofthe fluid containing these monomers display high molecular weightpolymeric properties.

EXAMPLE 6

1. All rheological, solubility etc. properties are a strong function ofthe alkyl length (n).

2. Dissolution of nonionic polymerizable (and nonpolymerizable moietiescan, in specific instances, markedly enhance the rheological propertiesof the aqueous cationic monomer viscoelastic fluids (FIG. 6).

EXAMPLE 7

1. Dissolution of nonionic polymerizable (and nonpolymerizable) moietiescan, in specific instances, markedly reduce the rheological propertiesof the aqueous cationic viscoelastic fluids (FIG. 7). It should benoted, however, that the rheological properties of the fluids are stillboth scientifically interesting and technologically useful. Apparently,the principal reason for the reduction in the solution properties is dueto the reduction in the solubility of either the individual monomerentities and/or the rod-like structures themselves.

We note at this point that the synthesis, preparation and utilization ofcationic polymerizable viscoelastic monomers and fluid compositions isuseful to impart improved and useful rheological properties to aqueousfluids. The specific properties can be varied over a very broad rangedue to the ability to readily modify or change a number of compositionalvariables in the monomer and/or counterion structure.

These types of monomer containing aqueous viscoelastic fluids are usefulas rheological control additives in a variety of drilling operations.Included in this category are a variety of well control and workoverfluids, fracturing fluids, brine viscosifiers, gelation agents, foamingadditives, completion fluids, enhanced oil recovery, drag reductionagents and the like.

EXAMPLE 8 Solution Behavior In High Ionic Strength Solutions

It is anticipated that the morphology, i.e. structures, that thesemonomers can assume in solution is quite complex and undoubtedly, thedynamics of these structures can change rapidly with time, temperature,stress, strain rate, pressure, ionic strength and the like. However, ourfindings strongly indicate that the monomers form relatively largerod-like structures. It is the large dimensions of the individual rodsand their interactions (i.e. charge-charge, purely geometrical-type rodoverlappings and the like) that primarily contributes to the unusualrheology of the viscoelastic fluids. It should be noted, however, thatthe complete verification of the solution morphology does not in any waydetract from the utilization of the unique properties of theseiscoelastic fluids containing the above specified polymerizable cationicmonomers.

FIGS. 8-16 show some representative rheological properties of the abovedescribed cationic monomer viscoelastic fluids. The solutions in thesespecific examples are either fresh or brine (sodium chloride and calciumchloride) solutions. An examination of the data clearly confirms thefollowing conclusions:

1. The rheological properties in fresh water of these cationic monomerswith salicylic acid-sodium salt as the counterion are a strong functionof the alkyl length, n (C₁₃ <<C₁₆ <<C₁₈).

2. At relatively low monomer concentrations, very dramatic viscosity isobserved. These values are a strong function of the alkyl length (FIGS.8-11).

3. Dissolution of acid, base and salt, can in specific instances,markedly enhance the rheological properties of the aqueous cationicviscoelastic fluid (FIGS. 12-16). The specific viscosity value change isa strong function of the alkyl length, n, in these cationic monomers,i.e. C₁₈ <<C₁₄ <<C₁₆. In fact, 106A displays a dramatic rise inviscosity with salt (NaCl) concentration. 106B and 106C shows areduction in viscosity in the same salt concentration range.

4. Similar observations as in 3 are noted in solutions containing highlevels of calcium chloride (FIGS. 15-16).

5. From the results it is clearly found that ion-exchange is not adominant factor, in these specific monomer solutions, in determiningstructural stability until very high levels of salt are reached.

Again in summary, the utilization of cationic polymerizable,viscoelastic monomers and fluid compositions is noted to impart improvedand useful rheological properties to fresh and high brine solutions. Thespecific properties, as noted within, can be varied over a broad rangedue to the ability to modify a number of compositional and structuralvariables in the monomer and/or counterion structure.

These types of monomers containing high brine viscoelastic fluids areuseful as rheological control agents in a variety of drilling andcompletion fluids, especially where high brine solutions are used orfound.

What is claimed is:
 1. A water soluble polymerizable monomercharacterized by the formula: ##STR4## wherein n is 6 to
 22. 2. Amonomer according to claim 1, wherein n is
 13. 3. A monomer according toclaim 1, wherein n is
 15. 4. A monomer according to claim 1, wherein nis 17.